The world number one's game collapsed in the second set and she was repeatedly left stranded by Pennetta's repertoire of lobs, volleys and powerful baseline returns.

Facing match point, Wozniacki staged a dramatic comeback to force a tie break in the third set but Pennetta superbly lobbed the flat-footed Dane to seal the match.

"I knew that it was going to be tough. I did my best today; it just wasn't good enough. I had my chances and didn't take them," said Wozniacki, who had been the only top-10 women's player left in the event. "Of course it's disappointing right now.

"I had 6-5, 30-love (in the third set) and I should have closed the match off. But...Flavia played some good points at that time, and I just made a few wrong decisions and the match changed," added the 21-year-old.

Former world number one Ana Ivanovic became the latest high-profile player to exit the tournament when the Serbian retired from her quarter-final against Agnieszka Radwanska with back problems.

The 23-year-old was a set down when the injury she sustained last week in Tokyo flared up.

Maria Sharapova and Serena Williams pulled out of the competition a week ago, Victoria Azarenka failed to show for her match on Thursday due to injury while home hope Li Na exited in the first round, Sam Stosur went out in the second and Vera Zvonareva the third.

In the men's draw, world number one and 2010 champion Novak Djokovic was forced to pull out because of a back injury.

Ivanovic said she had been fighting a losing battle all week.

"I have had lots of treatments on the injury, but it hasn't got much better," she said. "I've been fighting it the whole week. I had an MRI here yesterday. The doctors are still looking at the results.

"I'm talking to physios and just making sure about the next steps to see what's wrong and what's going on. It's too early to say anything."

Romanian Monica Niculescu came from behind to beat Maria Kirilenko 3-6 6-3 6-4 to clinch a semi-final berth while in the men's competition Tomas Berdych thrashed Spaniard Fernando Verdasco 6-1 6-0 to seal a place in the last four.

The Czech faces the winner of the late clash between Frenchman Jo-Wilfried Tsonga and Spain's Juan Carlos Ferrero.

Ivan Ljubicic also reached the last four with a 7-6 6-7 6-2 win over Mikhail Youzhny.

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